It is well known that freezing and thawing cycles can be extremely damaging to water-saturated hardened cement compositions such as concrete. The best known technique to prevent or reduce the damage done is the incorporation in the composition of microscopically fine pores or voids. The pores or voids function as internal expansion chambers and can therefore protect the concrete from frost damage by relieving the hydraulic pressure caused by an advancing freezing front in the concrete. The method used in the prior art for artificially producing such voids in concrete has been by means of air-entraining agents, which stabilize tiny bubbles of air that are entrapped in the concrete during mixing.
These air voids are typically stabilized by use of surfactants during the mixing process of wet cast concrete. Unfortunately, this approach of entraining air voids in concrete is plagued by a number of production and placement issues, some of which are the following:
Air Content—Changes in air content of the cementitious mixture can result in concrete with poor resistance to freezing and thawing distress if the air content drops with time or reduce the compressive strength of concrete if the air content increases with time. Examples are pumping concrete (decrease air content by compression), job-site addition of a superplasticizer (often elevates air content or destabilizes the air void system), interaction of specific admixtures with the air-entraining surfactant (could increase or decrease air content).
Air Void Stabilization: The inability to stabilize air bubbles can be due to the presence of materials that adsorb the stabilizing surfactant, i.e., fly ash with high surface area carbon or insufficient water for the surfactant to work properly, i.e, low slump concrete.
Air Void Characteristics: Formation of bubbles that are too large to provide resistance to freezing and thawing, can be the result of poor quality or poorly graded aggregates, use of other admixtures that destabilize the bubbles, etc. Such voids are often unstable and tend to float to the surface of the fresh concrete.
Overfinishing: Removal of air by overfinishing, removes air from the surface of the concrete, typically resulting in distress by scaling of the detrained zone of cement paste adjacent to the overfinished surface.
The generation and stabilization of air at the time of mixing and ensuring it remains at the appropriate amount and air void size until the concrete hardens are the largest day-to-day challenges for the ready mix concrete producer in North America.
Adequately air-entrained concrete remains one of the most difficult types of concrete to make. The air content and the characteristics of the air void system entrained into the concrete cannot be controlled by direct quantitative means, but only indirectly through the amount/type of air-entraining agent added to the mixture. Factors such as the composition and particle shape of the aggregates, the type and quantity of cement in the mix, the consistency of the concrete, the type of mixer used, the mixing time, and the temperature all influence the performance of the air-entraining agent. The void size distribution in ordinary air-entrained concrete can show a very wide range of variation, between 10 and 3,000 micrometers (μm) or more. In such concrete, besides the small voids which are essential to cyclic freeze-thaw resistance, the presence of larger voids—which contribute little to the durability of the concrete and could reduce the strength of the concrete—has to be accepted as an unavoidable feature.
The characteristics of an air void system in hardened concrete are determined by means of ASTM C457 Standard Test method for Microscopical Determination of Parameters of the Air-Void System in Hardened concrete. These characteristics are expressed as a series of parameters that are indicative of the average voids size (specific surface area), volumetric abundance (air content) and average distance between the voids (spacing factor). These values have been used in the concrete industry to determine the expected performance and durability of concrete in a water-saturated cyclic freezing environment. ACI guidelines recommend that the specific area be greater than 600 in−1 and the spacing factor equal to or less than 0.008 in to ensure resistance to freezing and thawing cycles.
Those skilled in the art have learned to control for these influences by the application of appropriate rules for making air-entrained concrete. The making of such concrete requires the exercise of particular care. Specifically, the air content must be checked regularly. If the air content is too low, the front resistance of the concrete will be inadequate. But, if the air content is too high, the compressive strength will be adversely affected.
The methods for controlling air voids in the prior art often result in inconsistent performance. If air bubbles of acceptable size and spacing are not entrained by the action of mixing, then no amount of bubble stabilizing chemical systems can produce an acceptable air void structure in the hardened concrete.
Therefore, it is desirable to provide an admixture which produces a freeze-thaw durable void structure directly in a wet cast cementitious mixture, without requiring the shear conditions for generation of properly sized air bubbles during mixing. The void structures may comprise optimally sized voids to the wet cast mixture that provide the cementitious composition with improved freeze-thaw durability. The admixture should also reduce or eliminate the reduction of compressive strength for products manufactured from wet cast mixtures containing conventional air-entraining chemical admixtures.